1. Field of the Invention
The present invention relates to a camshaft support structure for an internal combustion engine, and, in particular, to a camshaft support structure suitable for supporting camshafts in an internal combustion engine that is mounted on a vehicle.
2. Description of the Related Art
As described in Japanese Patent Application Publication No. 7-166956 (JP-A-7-166956), for example, a camshaft support structure in which upper bearings are integrated with a head cover is available. This conventional support structure is provided with a lower bearing that is paired with each upper bearing. The upper bearing and the lower bearing are both formed so as to have semicircular concave surfaces, and support camshafts by being fixed in a state where the upper and lower bearings are opposed to each other.
Cams for opening intake valves or exhaust valves are fixed to the camshaft. The intake valves and the exhaust valves are opened by being pushed by cam noses as the camshafts rotate. When the cam nose pushes and opens the intake valve or the exhaust valve, the reaction force is applied to the camshaft. The reaction force acts in a direction such that the camshaft is pressed against the upper bearings. For this reason, the upper bearings require higher rigidity than the lower bearings.
With the above conventional support structure, it is possible to give high rigidity to the upper bearings by integrating them with the head cover. Such rigidity makes it possible to support the camshaft with high accuracy owing to the sufficiently high rigidity.
In order to obtain the above conventional support structure, the assembly procedure as described below is necessarily taken. Specifically, in order to realize this structure, a cylinder head is first fitted with the intake valves and the exhaust valves. Next, rocker arms are placed on the intake valves and the exhaust valves. The rocker arms are the members for transmitting the pushing force of the cams to valve elements.
In another step than the above step, camshafts are fitted to the head cover. That is, in a state where the camshafts are placed on the upper bearings in the head cover, the lower bearings are fixed to the upper bearings. As a result, the camshafts are rotatably held by the head cover.
After the above step, the head cover is fixed to the cylinder head with the camshafts held by the head cover. In this step, it is necessary that the camshaft is disposed so that the rocker arms are properly sandwiched between the cams and the valve elements.
However, normally the rocker arms are merely put on the valve elements until they are held down by the camshafts. For this reason, the rocker arms are prone to fall off when the head cover holding the camshafts is fixed to the cylinder head. In order to prevent such falling off, it is preferable to be able to see the rocker arms when the head cover is fixed to the cylinder head. Moreover, in order to check the condition of the valve system, it is preferable to be able to easily see the condition of the rocker arms even after the internal combustion engine has been properly assembled.
However, with regard to the above conventional support structure, when the head cover is fixed to the cylinder head, it is very difficult to see the rocker arms that are located inside the head cover. In addition, it is necessary to remove the head cover in order to see the condition of the rocker arms; and it is impossible to see the rocker arms in a state where the assembly is completed. In this respect, the above conventional support structure is not always preferable from the viewpoint of productivity and easiness of maintenance/inspection.
The valve train of the internal combustion engine, especially the parts around the cams are important elements to allow the internal combustion engine to operate properly. For this reason, it is preferable to be able to check whether the cams are properly installed, or whether foreign matter is caught around the cams, for example, through visual inspection at the time of the maintenance/inspection of the internal combustion engine.
However, because the upper bearings are integrated with the head cover in the above conventional support structure, it is impossible to remove the head cover, leaving the camshafts properly held in the internal combustion engine. Accordingly, if this support structure is used, it is impossible to see the cams, leaving the camshafts held. In this respect, the above conventional support structure is not always ideal from the viewpoint of productivity in manufacturing the internal combustion engine, and the workability of the maintenance/inspection.